Adaptively controlled pre-distortion circuits for RF power amplifiers

ABSTRACT

A system includes a crest-factor reduction circuit, a signal analyzer, and a pre-distortion circuit. The crest-factor reduction circuit reduces a crest factor of a baseband signal and generates a feedforward signal. The signal analyzer generates parameters based on the feedforward signal and an output signal from a power amplifier. The pre-distortion circuit generates a pre-distorted baseband signal based on the parameters for input to the power amplifier.

CROSS REFERENCE TO RELATED APPLICATIONS

The present disclosure is a continuation of U.S. patent application Ser.No. 14/166,422 filed on Jan. 28, 2014. The entire disclosure of theapplication referenced above is incorporated herein by reference.

The present disclosure relates to U.S. patent application Ser. No.13/897,119, entitled “Crest Factor Reduction for Band-LimitedMulti-Carrier Signals,” filed on May 17, 2013, now U.S. Pat. No.8,937,993. The entire disclosure of the application referenced above isincorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to signal pre-distortion techniques inpower amplifiers. In particular, the present invention relates to signalpre-distortion techniques used in conjunction with an integrated signalanalyzer.

2. Discussion of the Related Art

Adaptive digital pre-distortion (DPD) is a technique widely used in themacro-cell base stations of “third-generation” (3G) wirelesscommunication systems. In a 3G macro-cell base station, a poweramplifier typically has 42˜48 dBm output power. In contrast, in “fourthgeneration” (4G) wireless communication systems, and beyond, small-cellbase stations are often expected, in which a typical power amplifier has27˜35 dBm output power. Because the output power of amplifiers in such asmall-cell base station is 10˜20 dB lower than macro-cell base stationsof a 3G base station, it is desirable to reduce the power consumption ofa pre-distortion circuit in a 4G small-cell base station by 10˜20 dB, soas to maintain overall power efficiency. In addition, it is alsodesirable to reduce the cost of the pre-distortion circuit by 10%, whichamounts to approximately 1% of the cost in a typical budget for amacro-cell base station.

In the prior art, adaptive digital pre-distortion techniques useexpensive high-speed and high-precision analog-to-digital converters(ADCs) to acquire accurate waveforms of the output signals of a poweramplifier (PA). As the non-linearity of a PA enhances the PA's outputbandwidth and as an intermediate-frequency down-conversion is requiredfor eliminating I/Q imbalance, the ADC's sampling rate in such adaptivedigital pre-distortion application is typically 10 or more times theoriginal RF bandwidth, typically between 200 to 1000 mega-samples (MS)per second for RF signals with 20˜100 MHz bandwidth (e.g. 240 MS/s for a4-carrier WCDMA signal). In addition, to detect an out-of-band emissionas low as −60 dBc, the ADC needs an effective number of bits (ENOB) ofapproximately 11. As a result, the prior art uses dedicated, stand-aloneADC integrated circuits with ENOB greater than 10 at 200˜1000 MS/s.However, such high-precision and high-speed ADC integrated circuits areexpensive and power-consuming.

In addition to the high-speed, high-precision dedicated ADC integratedcircuits having high power requirements, the signal processingalgorithms that are carried out in conjunction with the ADC are alsovery computationally complex and intensive. Consequently, fast,power-consuming digital signal processors (DSP) are required. Such DPDcircuits are too costly and require too much power to be suitable foruse in a small-cell base station.

Prior art analog RF pre-distortion methods can be low-power andlow-cost. However, existing analog RF pre-distortion circuits are noteasily adaptable for use in small-cell base station applications for tworeasons. First, the adjacent channel leakage ratio (ACLR) performance islimited by analog signal processing. Second, for small-cell base stationapplication, the task may require integrating an analog pre-distortioncircuit, which is typically designed for an older CMOS process (e.g.0.18-μm CMOS) onto a transceiver integrated circuit that is typicallydesigned for a newer CMOS process (e.g. 65-nm CMOS).

SUMMARY

The present invention shows a low-power, low-cost, adaptive digitalpre-distortion (DPD) for linearizing power amplifiers in small-cell basestations. The present invention does not require use of high-speed,high-precision ADCs.

According to one embodiment of the present invention, a pre-distortioncircuit for a power amplifier may include:

-   -   (a) a digital interface for receiving a complex-value digital        baseband signal;    -   (b) a mixed-signal pre-distortion circuit adaptively controlled        by a set of parameter values, receiving the complex-value        digital baseband signal to provide a pre-distorted signal for        input to the power amplifier, the mixed-signal pre-distortion        circuit may include:        -   (i) a digital pre-distortion circuit that transforms the            complex-value digital baseband signal to a pre-distorted            complex-value digital baseband signal by applying to the            complex-value digital baseband signal a memory-less            non-linear gain and a polynomial function based on the            complex-value digital baseband signal and one or more            delayed copies of the complex-value digital baseband signal;            and        -   (ii) an up-converter that converts the pre-distorted            complex-value digital baseband signal to the pre-distorted            signal; and    -   (c) a signal analyzer receiving an output signal from the power        amplifier to provide the parameter values from time to time to        the mixed-signal pre-distortion circuit.

In one embodiment, the polynomial function may be composed of one ormore powers of the moduli of the complex-value digital baseband signaland one or more powers of the moduli of the delayed copies of thecomplex-value digital baseband signal, and the memory-less non-lineargain is applied after a unit delay to the complex-value digital basebandsignal. In addition, the memory-less non-linear gain may be a polynomialfunction of the complex value digital baseband signal.

In one embodiment, the digital pre-distortion circuit uses look-uptables to compute either one or both of the memory-less non-linear gainand the polynomial function.

The digital pre-distortion circuit may include an over-sampler thatincreases the data rate in the complex-value digital baseband signal byat least a factor of 2.

The signal analyzer may include (a) a quadrature down-converter thatconverts the output signal from the power amplifier to a complex-valuefeedback signal that is at baseband or at a near-baseband intermediatefrequency; (b) an analog-to-digital converter that converts thecomplex-value feedback signal to a first digital complex-value feedbacksignal; and (c) a digital signal analysis circuit that receives thedigital complex-value feedback signal to provide the parameter values.The up-converter and the quadrature down-converter operate from a commontiming signal. The digital pre-distortion circuit and the signalanalyzer may be implemented using one or more digital signal processors.The memory-less non-linear gain is computed based on a magnitude offirst digital complex-value feedback signal.

According to one embodiment of the present invention, the signalanalyzer may receive in addition to the pre-distorted signal. In thatembodiment, the signal analyzer may further include (a) a secondquadrature down-converter that converts the pre-distorted signal to asecond complex-value feedback signal that is at baseband or at thenear-baseband intermediate frequency; and (b) an analog-to-digitalconverter that converts the second complex-value feedback signal to asecond digital complex-value feedback signal. The digital signalanalysis circuit in that case provides the parameter values based onboth the first digital complex-value feedback signal and the seconddigital complex-value feedback signal. The digital signal analysiscircuit may perform one or more of DC offset correction, I/Q imbalancecorrection and delay matching on either or both of the first digitalcomplex-value feedback signal and the second digital complex-valuefeedback signal. The digital signal analysis may also perform lineardistortion correction on the first complex-value feedback signal. Thedigital signal analysis circuit may compute the parameter values basedon minimizing a difference between the first complex-value feedbacksignal and the second complex-value feedback signal.

According to one embodiment of the present invention, the pre-distortioncircuit may also include a crest-factor reduction circuit that reduces acrest factor in the complex-value digital baseband signal provided tothe mixed-signal pre-distortion circuit. The signal analyzer may receivealso a complex-value feed-forward signal representative of thecomplex-value digital baseband signal, which may be the crestfactor-reduced complex-value digital baseband signal. In one of theseembodiments, the signal analyzer may include (a) a quadraturedown-converter that converts the output signal from the power amplifierto a complex-value feedback signal that is at baseband or at anear-baseband intermediate frequency; (b) an analog-to-digital converterthat converts the complex-value feedback signal to a first digitalcomplex-value feedback signal; and (c) a digital signal analysis circuitthat receives the digital complex-value feedback signal to provide theparameter values. The up-converter and the quadrature down-converteroperate from a common timing signal. The digital signal analysis circuitmay provide the parameter values based on both the first digitalcomplex-value feedback signal and the complex-value feed-forward signal.The digital signal analysis circuit may perform one or more of I/Qbalance correction, delay matching, complex gain adjustments, frequencyoffset correction and DC offset correction on the complex-valuefeed-forward signal. The digital signal analysis circuit may compute theparameter values based on minimizing a difference between the firstcomplex-value feedback signal and the complex-value feed-forward signal.The digital signal analysis may perform frequency offset correction onthe complex-value feed-forward signal.

According to one embodiment of the present invention, the pre-distortioncircuit may include an analog-to-digital converter enhancer circuit thatreceives from the digital signal analysis circuit a data signalrepresenting one or more clock periods of the complex value feed-forwardsignal. The analog-to-digital converter enhancer circuit may apply acurrent-steering digital-to-analog technique. The analog-to-digitalconverter enhancer circuit may include (a) a digital-to-analog circuitthat converts the data signal to a complex data current; and (b) acurrent combination circuit that provides an output currentrepresentative of a difference between the complex data current and thecomplex-value feedback signal. The analog-to-digital converter enhancercircuit may further include a resistor element that converts the outputcurrent into an output voltage. The analog-to-digital converter enhancercircuit may further include a second stage that receives the outputvoltage and a second data signal from the digital signal analysiscircuit, the second stage applies a charge-redistribution technique. Thesecond stage may include (a) cascaded first and second tracking-and-holdcircuits operated by complementary clock signals to provide held voltagesignal; (b) a digital-to-analog converter that converts to second datasignal to provide an voltage data signal; and (c) a voltage summer thatprovides a second-stage output signal representative of a voltagedifference between the held voltage signal and the voltage data signal.

According to one embodiment of the present invention, the mixed signalpre-distortion circuit in the pre-distortion circuit may be integratedinto a transceiver integrated circuit, while the signal analyzer circuitis integrated into an integrated circuit separate from the transceiverintegrated circuit. Alternatively, the digital pre-distortion circuitand the digital signal analysis circuit may be integrated into amixed-signal integrated circuit, while the up-converter is integratedinto an integrated circuit separate from the mixed-signal integratedcircuit.

According to one embodiment of the present invention, a signal analyzermay be provided that has a frequency synthesizer, a quadraturedown-converter, ADCs, and other digital circuits integrated on a CMOSintegrated circuit that can be fabricated by a standard mixed-signalCMOS process. Consequently, the present invention greatly reduces therequirement on analog-to-digital conversion speed and precision in theADCs of the signal analyze, thereby obviating a need for costly,stand-alone ADC integrated circuits. In conjunction the signal analyzer,the present invention provides pre-distortion and adaptation algorithmsthat achieve low power operations.

The present invention is better understood upon consideration of thedetailed description below, in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of system 100, according to one embodiment ofthe present invention.

FIG. 2 is block diagram 200 representing a model of the operationscarried out in DPD processor 106, according to one embodiment of thepresent invention.

FIG. 3 shows arithmetic-logic circuit 300 in one implementation of DPDprocessor 106 according to low-complexity model 200 of FIG. 2.

FIG. 4 is a block diagram of system 400, according to one embodiment ofthe present invention.

FIG. 5 is a block diagram of digital waveform matching circuit 500,which implements a method for measuring relative waveform distortion,according to one embodiment of the present invention.

FIG. 6 is a block diagram of system 600, in accordance with oneembodiment of the present invention.

FIG. 7 shows waveform matching circuit 700, which performs DC offsetcorrection, I/Q imbalance correction, frequency offset correction,linear distortion correction, delay matching, and complex gain matching,in accordance with one embodiment of the present invention.

FIG. 8 shows system 800, which CFR processor 104, DPD processor 106, andsignal analyzer 612, quadrature down-converter 110 are integrated into asingle integrated circuit, in accordance with one embodiment of thepresent invention.

FIG. 9 is a block diagram of system 900, including enhanced ADC 911, inaccordance with one embodiment of the present invention.

FIG. 10 is a block diagram showing enhanced ADCs 1011 a and 1011 b forthe down-converted in-phase and quadrature analog signals derived fromRF feedback signal 115, respectively, in accordance with one embodimentof the present invention.

FIG. 11 shows waveform-transform circuit 1100, providing complex-valueddigital signal D_(I)+jD_(Q), in accordance with one embodiment of thepresent invention.

FIG. 12 is a block diagram showing two-stage ADC-enhancer circuit 1200,in accordance with one embodiment of the present invention.

To simplify the detailed description below and to allow cross-referenceamong the figures, like elements are assigned like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of system 100, according to one embodiment ofthe present invention. System 100 integrates crest-factor reduction(CFR) and digital pre-distortion (DPD) operations on a transceiverintegrated circuit that is represented by circuit block 101 of FIG. 1.The transceiver circuit may be implemented using a mixed-signal CMOSprocess (e.g. 65-nm CMOS at this time). As shown in FIG. 1, basebanddata source 103 provides a complex baseband signal to be transmitted(e.g., a 4-carrier WCDMA or 20-MHz LTE signal). Circuit block 101, whichreceives the baseband signal data and signal parameters 118 (not shown;provided over SPI Bus 113) from a signal analyzer circuit 102, includesCFR processor 104, double sampler 105, DPD processor 106 anddigital-to-RF up-converter 107. CFR processor 104 reduces thepeak-to-average power ratio (PAR) of the input complex baseband signal.The output signal of CFR processor 104 may have a sampling rate of 34times the bandwidth of the complex baseband signal (e.g. 61.44 MS/s fora 4-carrier WCDMA or 20-MHz LTE signal). The sampling rate is doubled bydouble sampler 105 to 68 times the signal bandwidth of the complexbaseband signal (e.g. 122.88 MS/s for a 20-MHz LTE) before beingprovided to DPD processor 106. The output signal from digital-to-RF 107is the analog signal to be transmitted that is modulated on one or moreRF carrier signals and pre-distorted for linearity in power amplifier108. CFR processor 104 may implement any of a number of crest factorreduction techniques, e.g., the crest factor reduction techniquesdisclosed in copending U.S. patent application (“CopendingApplication”), Ser. No. 13/897,119, entitled “Crest Factor Reduction forBand-Limited Multi-Carrier Signals,” filed on May 17, 2013. Thedisclosure of Copending Application is hereby incorporated by referencein its entirety.

The output signal 115 of power amplifier 108 is fed back in system 100through another CMOS integrated circuit, represented by circuit block102. Circuit block 102 represents a signal-analyzer integrated circuit,which is used for adaptive control of DPD operations of system 100. Asshown in FIG. 1, circuit block 102 includes quadrature down-converter110, which down-converts the feedback signal 115. The down-convertedanalog signal is converted into digital representation (i.e., digitalsignal 117) using ADCs 111. Digital circuit 112 analyzes digital signal117 to provide signal parameters 118. Unlike circuit block 101, circuitblock 102 may be implemented using a less expensive CMOS process, andmay also include other circuit blocks (e.g. a circuit for controllingthe bias voltages of power amplifier 108). Communication between thecircuits of circuit blocks 101 and 102 may also be conducted overindustry standard data communication protocol, such as that implementedin an industry standard SPI bus (represented by SPI bus 113 in FIG. 1).

FIG. 2 is block diagram 200 representing a model of the operationscarried out in DPD processor 106, according to one embodiment of thepresent invention. As shown in FIG. 2, DPD processor 106 receives asequence of complex samples {x_(k)} and outputs a complex sequence{y_(k)}. The discrete-time input-output relation of DPD processor 106may be represented by:y _(k) =x _(k−l) G(|x _(k−l) |;v ₁)+P(x _(k) ,x _(k−1) , . . . ,x _(k−N);v ₂)  (1)where xG(|x|; v₁) is a memory-less non-linearity (with vector parameterv₁ represented by block 201 in FIG. 2. The memory-less non-linearity ofblock 201 is applied to a copy of the input signal delayed in delayelement 202 by £ samples, N is the memory span of DPD processor 106(e::; N) and P is a non-linear function (with vector parameter v₂) ofN+1 complex variables, which is represented by block 203 in FIG. 2.Vector parameters v₁ and v₂ may be, for example, some of signalparameters 118 received from the signal analyzer of circuit block 102 ofFIG. 1. Memory-less non-linearity of block 201 may be seen as a complexgain which depends only on the instantaneous amplitude of the inputsignal.

The non-linear function P(x_(k),x_(k−1), . . . , x_(k−N); v₂) of block203, also referred to as the “memory kernel”, mitigates memory effectsin power amplifier 108, which can impose a fundamental limit on theperformance of DPD processor 106, as such memory effects are difficultto fully compensate for. The memory kernel may be implemented using manypossible algorithms, such as memory polynomials, discrete-time Volterraseries, and artificial neural networks. DPD processors in the prior artare typically complicated datapath processors because of the complexityof the algorithms implemented (e.g. discrete-time Volterra series with alarge number of terms, and adaptation algorithms with extremely highcomputational complexity). Complex adaption algorithms include, forexample, parameter estimations of complicated behavior models). Thepresent inventors discover that the computation complexity of both theDPD processor 106 and the adaptation algorithms may be reduced.According to one embodiment of invention, DPD processor 106 mayimplement the following input-output relation:

$\begin{matrix}{y_{k} = {{x_{k - 1}{G\left( {{x_{k - 1}};v_{1}} \right)}} + {j\;{c_{0}\left( {x_{k} - x_{k - 2}} \right)}} + {\quad{{x_{k - 1}\left( {{c_{1}{x_{k}}} + {c_{2}{x_{k}}^{2}} + {c_{3}{x_{k}}^{4}} + {c_{4}{x_{k - 2}}} + {c_{5}{x_{k - 2}}^{2}} + {c_{6}{x_{k - 2}}^{4}}} \right)} + {x_{k}\left( {{c_{1}{x_{k}}} + {c_{8}{x_{k}}^{2}} + {c_{9}{x_{k}}^{4}} + {x_{k - 2}\left( {{c_{10}{x_{k - 2}}} + {c_{11}{x_{k - 2}}^{2}} + {c_{12}{x_{k - 2}}^{4}}} \right)}} \right.}}}}} & (2)\end{matrix}$where j=√{square root over (−1)}, c₀ is a real number and the c,coefficients, 1≦n≦N, are complex values. Laboratory experiments showthat the low-complexity equation (2) can be implemented in astraightforward manner to deliver a surprisingly high performance over awide range of power amplifiers, including those power amplifiers thatare based on LDMOS, GaN, SiGe-HBT, GaAs and other types of transistors.

FIG. 3 shows arithmetic-logic circuit 300 in one implementation of DPDprocessor 106 according to low-complexity model 200 of FIG. 2. In fact,as shown in FIG. 3, arithmetic-logic circuit 300 may be used toimplement equation (2) above. Arithmetic-logic circuit 300 includesamplitude detector 319 which provides the absolute value of eachincoming digital sample received from input terminal 301. The absolutevalues of digital samples at various delays are used, for example, inthe non-linear gain and the polynomial terms of Equation (2).

Each digital sample may be represented, for example, by a fixed-pointcomplex number in rectangular coordinate form. As some complex numbercomputations may be simpler if carried out in polar coordinate form,amplitude detector 319 may use the CORDIC algorithm to convert a complexnumber from rectangular coordinate form to polar coordinate form.Computing in rectangular coordinates form, amplitude detector 319 may beimplemented using multipliers and a fixed-point square-root algorithm,the absolute value of x=x_(I)+jx_(Q) being √{square root over (x_(I)²+x_(Q) ²)}.

One way to calculate the square root takes advantage that a positivefixed-point number n may be represented in the form n=2^(m)(1+α) where mis an integer and a is a fixed-point number satisfying 0≦α<1. One canapproximate log₂ n by the function q log 2(n)=m+α. The logarithm (base2) of the square-root of n may then be expressed as

$\left. {{\frac{1}{2\;}\log\; 2(n)} = {{\frac{1}{2}m} + \alpha}} \right).$The square root can be approximated by

$2^{m/2}\left( {1 + {\frac{1}{2}\alpha}} \right)$for an even m, or

${2^{{({m - l})}/2} \times 1.414\left( {1 + {\frac{1}{2}\alpha}} \right)},$for an odd m. The digital circuit for the q log 2( ) function can bevery simply implemented. Alternately, a polynomial fit may be used tocalculate √{square root over (1+α)} (e.g. 1.0013+0.4821α−0.0702α²). Thesquare root for n using the polynomial fit is 2^(m/2)(1.0013+0.4821α−0.0702α²) for an even m, or2^((m−1)/2)×1.414(1.0013+0.4821α−0.0702α²), for an odd m.

Returning to arithmetic-logic circuit 300 of FIG. 3, delay elements 314and 316, multiplier 305, and non-linear gain element 311 togetherimplements the non-linear gain term x_(k−1)G(|x_(k−1)|; v₁). Non-lineargain element 311 may be implemented by a combination of look-up table(LUT) together with one-dimensional linear interpolation betweenadjacent table elements. Under this approach, the LUT circuit looks up apredetermined number of the most-significant-bits of its input value,and then adds to it the value of the remaining bits, which is obtainedby linear interpolation. Thus, such a LUT circuit provides thepiece-wise linear approximation of smooth non-linear functions.

As shown in FIG. 3, multiplier 318 and polynomial element 317 implementspolynomial term x_(k)P₃(|x_(k)|). Delay elements 313, 314, 315 and 316,multiplier 307, and polynomial element 309 implements polynomial termx_(k−2)P₄(|x_(k−2)|). Delay elements 315 and 316, multiplier 305, adder308 and polynomial elements 310 and 312 implements polynomial polynomialterm x_(k−1)(P₁(|x_(k)|)+P₂(|x_(k−2)|)). Polynomial elements 309, 310,312, 317 may each implement, for example, a 4th-order polynomialfunction such as c1r+c2r2+c3r4. Each polynomial element may beimplemented, for example, by adders, multipliers or LUT circuits or anycombination of some or all of these elements.

As mentioned above, as shown in FIG. 1, the adaptive control of DPDprocessor 106 is based on RF feedback signal 115, which is obtained from30-dB coupler 109 at the output terminal of power amplifier 108. (RFfeedback signal 115 may be attenuated before being provided to signalanalyzer integrated circuit 102. RF feedback signal 115 is the outputsignal of power amplifier 108 together with some possible interference.In a multiple-input multiple-out (MIMO) system, for example, suchinterference may arise from multi-antenna coupling. The interference atthe output terminal of coupler 109 is an antenna interference attenuatedby the isolation ratio (>30 dB) of coupler 109. Typically, thesignal-to-interference ratio in the feedback path of RF feedback signal115 is better than 40 dB.

RF feedback signal 115 is converted to analog in-phase and quadraturesignals by quadrature down-converter 110 in signal analyzer integratedcircuit 102. Local oscillator (LO) signal 114 for down-converter 110 maybe provided by a PLL-based tunable frequency synthesizer that has afrequency resolution of ˜0.1 MHz. The down-converter synthesizer isadjusted to approximately the center frequency of baseband RF signalfrom baseband data source 103. The output I/Q signals of quadraturedown-converter 110 are converted to digital signals by ADCs 111, whichmay be conventional pipelined ADCs with 12-bit resolution and a˜10.5-bit ENOB. The clock rate of ADCs 111 may be variable and adjustedto roughly 4.5 times the signal bandwidth, e.g. 80100 MS/s for 20-MHzLTE.

Digital signal analyzer 112 may use fast Fourier transform (FFT) and theWelch's algorithm to measure the power spectrum density (PSD) of RFfeedback signal 115. Within signal analyzer integrated circuit 102,digital circuit block 112 may include a circuit for computing a256-point FFT. From a PSD analysis, signal analyzer integrated circuit102 obtains an estimate of the out-of-band emission power of RF feedbacksignal 115. Digital circuit block 112 may also include amicro-controller that runs a stochastic optimization algorithm (see,e.g., the disclosure of U.S. Pat. No. 8,136,081). Such an optimizationstep adjusts the control parameters of DPD processor 106 and minimizesout-of-band emission power. Serial Peripheral Interface (SPI) bus 113exchanges information between signal analyzer integrated circuit 102 andthe transceiver integrated circuit.

In one embodiment, coefficient c₀ in Equation (2) above is not adjustedfrom minimizing the out-of-band emission power of RF feedback signal115. Coefficient c₀ controls a linear filtering effect that cancompensate for non-flat gain across the bandwidth of the input RFsignal.

The LUT circuit in non-linear gain element 311 may be configured as apiece-wise linear approximation of the polynomial G(r)=Σ_(n=1)¹⁰a_(n)r^(n), with a_(n) being complex-valued coefficients. For manypower amplifiers, function G(r) may be further simplified to:G(r)=a ₁ r+a ₂ r ² +a ₄ r ⁴ +a ₆ r ⁶ +a ₈ r ⁸Such a polynomial representation reduces the degree of freedom ofcontrol parameters for DPD adaptation. This adaptation method is basedon finding the set of parameter values that minimize the out-of-bandemission power resulting from the non-linear distortion in RF feedbacksignal 115. Out-of-band emission is insensitive to I/Q imbalance inquadrature down-converter 110 and other sources of interferences, suchas the interference due to multi-antenna coupling in a MIMO system.

FIG. 4 is a block diagram of system 400, according to a secondembodiment of the present invention. As shown in FIG. 4, in addition toRF feedback signal 115 (RFFB), signal analyzer integrated circuit 402receives power amplifier input signal 411 (RFIN). Complex basebanddigital signals are obtained from RF feedback signal 115 and poweramplifier input signal 411 using quadrature down-converters 412 a and412 b. Quadrature down-converters 412 a and 412 b may be provided thesame LO signal 114. The down-converted feedback signals are converted todigital domain in ADCs 111, ADCs 111 are shown in FIG. 4 as ADCs 413 aand 413 b. Signal analyzer integrated circuit 402 measures not only thepower spectral density (PSD) of each input signal, but also the relativewaveform distortion between RF feedback signal 115 and power amplifierinput feedback signal 410. Using a signal analyzer that operates on anoutput signal of a power amplifier so as to control linearity of thepower amplifier is taught, for example, in U.S. Pat. No. 8,145,150.

Relative waveform distortion may be measured in signal analyzerintegrated circuit 402 using a digital waveform-matching subsystem, suchas waveform-matching circuit 500 illustrated by block diagram in FIG. 5.(Although waveform matching circuit 500 is described herein as acircuit, the operations carried out in waveform matching circuit 500 mayalso be implemented, entirely or in part, in software executable on amicroprocessor). Waveform matching circuit 500 corrects for DC offset,I/Q imbalance, and linear distortion in both RF feedback signal 115 andpower amplifier input feedback signal 410. As shown in FIG. 5, waveformmatching circuit 500 provides DC offset corrections to RF feedbacksignal 115 and power amplifier feedback signal 410 in DC offsetcorrection circuits 501 a and 501 b, respectively. The DCoffset-corrected signals are then corrected for I/Q imbalance in I/Qimbalance correction circuits 502 a and 502 b, respectively. Thefollowing method may be used for correcting I/Q imbalances:

-   -   (a) Detuning the LO frequency of a quadrature down-converter        (e.g., down-converter 412 a or 412 b) from the center frequency        of RF signals (e.g. RF feedback signal 115 or power amplifier        input feedback signal 114) by an amount equal to or slightly        higher than half of the RF signal bandwidth;    -   (b) measuring the PSD that shows the mirror-leakage power due to        I/Q imbalance; and    -   (c) adjusting the I/Q imbalance parameters of waveform matching        circuit (e.g., I/Q imbalance correction circuit 502 a or 502 b)        to minimize the mirror-leakage power.

FIG. 5 also shows complex gain and delay matching by finite impulseresponse (FIR) filter 503 a and integer delay circuit 503 b in thesignal paths of RF feedback signal 115 and power amplifier inputfeedback signal 410, respectively. FIR filter 503 a includes acomplex-coefficient FIR filter that provides, not only delay matching,but also linear distortion correction. Complex gain and delay matchingmay be accomplished, for example, using the techniques disclosed in U.S.Pat. No. 8,295,394. Waveform matching circuit 500 provides matchedcomplex signals (A_(k) and B_(k)), in which signal B_(k) is the complexgain and delay matched output signal of FIR filter 503 a, weighted by acorrelation between the output signal of FIR filter 503 a and the outputsignal A_(k) of delay circuit 503 b. Error signal A_(r) B_(k) isprovided by summer 505. An optimum matching is achieved by minimizingmean square error detected at mean square error detector 507 of theerror signal A_(k)−B_(k) (i.e. the average power of |A_(k)−B_(k)|²).

From statistical samples of A_(k)/B_(k), signal analyzer 414 measuresthe conditional expectations:ξ_(m) =E{(A _(k) /B _(k))|r _(m) <|B _(k) |<r _(m) +Δr}, for r_(m)=(m−0.5)Δr,m=1,2, . . .  (3)where the selected step size Δr may be a fraction (e.g., 1/32) of theenvelope peak. Using the calculated conditional expectations, the LUTcircuit of non-linear gain element 311 in DPD processor 106 areprogrammed to contain:G(mΔr)  (4)The stored values in the LUT circuits are monitored and updated fromtime to time. The memory kernel coefficients (except for c₀) arecontrolled to minimize the out-of-band emission in RF feedback signal115.

According to one embodiment of the present invention, signal analyzerintegrated circuit 102 may be implemented, for example, by the RFPALproduct from Scintera Corporation, Santa Clara, Calif. in conjunctionwith a microcontroller (e.g., Intel 8051). Signal analyzer integratedcircuit 102 carries out the following basic functions:

-   -   (a) monitoring the average power and power spectrum of RF        feedback signal 115 (RFFB) and power amplifier input signal 410        (RFIN);    -   (b) waveform matching between the RFIN and RFFB signals;    -   (c) monitoring the transmitter LO leakage that causes a spectral        line on the power spectrum;    -   (d) monitoring the transmitter I/Q imbalance from the mutual        correlation and average-power imbalance between the I/Q        components; and    -   (e) monitoring the power amplifier status, such as bias voltages        and ambient temperatures.

FIG. 6 is a block diagram of system 600, in accordance with oneembodiment of the present invention. In system 600, in addition toreceiving RF feedback signal 115 and exchanging data and control signalsgenerally over SRI bus 113, signal analyzer integrated circuit 602 alsoreceives from transceiver circuit block 101 output data signal 618 ofCFR processor 104, and local oscillator (LO) reference timing signal614. Up- and down-converters in system 600, i.e., up-converter indigital-to-RF up-converter 107 and quadrature down-converter 110, may becontrolled by common LO reference timing signal 614. LO reference timingsignal 614 may be obtained, for example, from a crystal oscillator fromthe up-converter synthesizer in digital-to-RF up-converter 107. Sharingreference timing signal 614 between digital-to-RF up-converter 107 andquadrature down-converter 110 allows phase synchronization between thefrequency synthesizers. Quadrature down-converter 110 has a synthesizerthat is normally adjusted to the nominal center frequency of RF feedbacksignal 115 plus a small frequency offset. The frequency offset resultsfrom a coarser frequency resolution in the synthesizer of quadraturedown-converter 110, relative to the frequency resolution in thesynthesizer the up-converter in digital-to-RF up-converter 107. Outputdata signal 618 of CFR processor 104 also shares a timing signal withADCs 111.

Digital signal analyzer 612 of FIG. 6 receives both digital data signal618, which is the crest factor-reduced complex baseband input signalfrom data source 103, and digital signal 117, which is the digitalrepresentation from ADCs 111 of down-converted RF feedback signal 115.In addition to PSD measurement, digital signal analyzer 612 provideswaveform matching between digital data signal 618 and digital signal117. FIG. 7 shows waveform matching circuit 700, which performs DCoffset correction on digital signal 117 (block 701), I/Q imbalancecorrection on both digital data signal 618 and digital signal 117(blocks 702 a and 702 b, respectively), frequency offset correction,linear distortion correction, delay matching, and complex gain matching(FIR filter block 703 and frequency correction block 704), in accordancewith one embodiment of the present invention. (Although waveformmatching circuit 700 is described herein as a circuit, the functions ofwaveform matching circuit 700 may also be performed, entirely or inpart, by software executable in a microprocessor or a digital signalprocessor.) Waveform matching circuit 700 generates matched signalsD_(k) and B_(k), which are respectively the output signal of frequencycorrection block 704 and the output signal of I/Q compensationcorrection block 702 a, weighted by a correlation between signal DK andthe output signal of I/A imbalance correction block 704. A differencebetween the two matched signals is taken at summer 505. Optimum matchingis achieved by minimizing mean square error (i.e., the average power of|D_(k)−B_(k)|²).

From statistical samples of D_(k)/B_(k), signal analyzer 612 measuresthe conditioned expectations:ζ_(m) =E{(D _(k) /B _(k))|r _(m) <|B _(k) |<r _(m) +Δr}, for r_(m)=(m−0.5)Δr,m=1,2 . . .  (5)The table data in the LUT circuits for non-linear gain model 311 of DPDprocessor 106 are updated toG ^((new))(mΔr)=ζ_(m) ·G ^((old))(mΔr)  (6)

The memory kernel coefficients (except for c₀) in DPD processor 106 arecontrolled to minimize the out-of-band emission of the feedback signal.

One advantage of the adaptation methods of the present invention allowsthe sampling rate of RF feedback signal 115 to be reduced to only 34times the signal bandwidth (e.g. 61.44 MS/s for 20-MHz LTE). Therefore,transceiver integrated circuit 101 may send digital data signal 618 todigital signal analyzer 612, instead of the double-sampled signal (i.e.,the input signal to DPD processor 106). In contrast, conventional DPDmodels for the power amplifiers must use the double-sampled signal forparameter estimation. Because the methods can take advantage of a lowerADC rate, signal analyzer 612 may be substantially simpler thanconventional signal analyzers for DPD applications.

FIG. 8 shows system 800, which CFR processor 104, DPD processor 106, andsignal analyzer 612, quadrature down-converter 110 are integrated into asingle integrated circuit, in accordance with one embodiment of thepresent invention. As shown in FIG. 8, Digital-to-RF up-converter 107may be integrated into a transceiver integrated circuit (e.g.,transceiver integrated circuit 802), which may be manufactured using aprevailing CMOS circuit manufacturing technology (e.g. 65-nm or 45-nmCMOS in year 2013). At the same time, the digital circuits, such as CFRprocessor 104, DPD processor 106, and signal analyzer 612, quadraturedown-converter 110, may be manufactured using an older CMOS process(e.g. 0.13-μm CMOS) to take advantage the trade-off between developmentcost and power consumption, as CFR and DPD algorithms applicable to thepresent invention are low complexity and high performance.

FIG. 9 is a block diagram of system 900, including enhanced ADC 911, inaccordance with one embodiment of the present invention. Enhanced ADC911 provides a higher analog-to-digital conversion rate and accuracy ata lower power than conventional pipelined ADC. As shown in FIG. 9,system 900 is similar to system 600 of FIG. 6, with enhanced ADC 911receiving digital signals 913 from digital signal analyzer 912. Digitalsignals 913 includes digital signals D_(k,I) and D_(k,Q), which are usedto enhanced conversion rate and accuracy in analog-to-digital conversionfor the down-converted quadrature signals derived from RF feedbacksignal 115.

FIG. 10 is a block diagram showing enhanced ADCs 1011 a and 1011 b forthe down-converted in-phase and quadrature analog signals derived fromRF feedback signal 115, respectively. The down-converted in-phase andquadrature analog signals derived from RF feedback signal 115 are shownin FIG. 10 as analog signals 1014 a and 1014 b. Enhanced ADCs 1011 a and1011 b each include an ADC enhancer circuit, shown in FIG. 10 as ADCenhancer circuits 1011 a and 1011 b, respectively. ADC enhancer circuits1011 a and 101 b receive signals D_(m) and D_(k,Q) and analog signals1014 a and 1014 b to provide output signals 1015 a and 1015 b toconventional ADCs 111 a and 111 b, respectively. (Conventional ADC 111 aand 111 b may be implemented using the same circuits in ADC 111 of FIGS.1, 4 and 6). Typically, quadrature down-converter 110 providescurrent-mode output for analog signals 1014 a and 1014 b. ADC enhancercircuits 1012 a and 1012 b use a current-steering digital-to-analogconverter (DAC) technique. From input analog current signal V(t) (i.e.,analog signals 1014 a or 1014 b) and digital signal D_(k) (i.e., D_(k,I)or D_(k,Q)), ADC enhancer circuit 1012 a or 1012 b generates an analogsignal V(t)−D(t) through the current sum. In FIG. 10, currentD(t)=Σ_(k)D_(k)u(t−kT) is the output current of DAC 1016 a or 1016 b,where u(t) is a rectangular pulse with one clock-period duration. TheV(t)−D(t) current is converted to analog voltage by load impedance 1017a or 1017 b, followed by amplification in voltage amplifier 1018 a or1018 b. Signals in ADC enhancer circuits 1012 a and 1012 b arepreferably implemented as differential signals.

A front-end circuit in ADC 111 a or 111 b is a switching capacitortrack-and-hold (T/H) circuit driven clock signal CLK. The clock rate(“sub-ADC sampling rate”) may be a multiple of the data rate of digitalsignal D_(k). To simplify the following discussion, the sub-ADC samplingrate is provided to be the same as the data rate of digital signalD_(k). ADC 111 a and 111 b provides digital signal S_(k,I) and S_(k,Q).S_(k) (i.e., S_(k,I) or S_(k,Q)) is given by:S _(k+τ) =A·[V(t _(k))−D _(k) δ]+q _(k)  (7)where τ is an integer delay, A is the gain of ADC enhancer circuit 1012a or 1012 b, δ is a DC offset, and q_(k) is a residual error dominatedby the quantization error of ADC 111 a or 111. The (A,δ) parameters ofADC enhancer circuits 1012 a or 1012 may be accurately identified from acalibration by turning off analog input current V(t).

The complex-valued digital signal D_(I)+jD_(Q), is generated fromwaveform-transform circuit 1100 shown in FIG. 11. Waveform transformcircuit 1100 receives a copy of digital data signal 618 from CFRprocessor 104. Digital data signal 618 may be represented as the valueC_(I)+jC_(Q). Waveform-transform circuit 1100 first performs an I/Qimbalance correction (block 1101), FIR filtering (block 1102, for delaymatching and linear distortion correction), complex gain adjustment(mixer 1103), frequency offset correction (mixer 1104), and DC offsetcorrection (summer 1105). To provide frequency offset correction, thegain-adjusted complex-valued signal is multiplied with e^(−j2π(Δf)k).For nearly-zero-IF down-conversion with a small frequency offset, thedata rate of the D_(I)+jD_(Q) equals to the data rate of digital datasignal C_(I)+jC_(Q), which is 34 times the original RF bandwidth (e.g.61.44 MS/s for 20-MHz LTE).

Analog amplifiers in ADC enhancer circuits 1011 a and 1011 b may beprovided by open-loop amplifiers without precise gain control and may bedesigned to have selectable low, medium, and high-gain modes. Forexample, the enhancer gain is 0, 15, and 30 dB. The enhanced ADCs startoperation from the following procedure:

-   -   (a) switching the ADC enhancer circuits 1011 a or 1011 b to a        low-gain mode;    -   (b) Using a stochastic optimization algorithm (e.g. any suitable        one of the algorithms disclosed in U.S. Pat. No. 8,136,081) to        adjust the control parameters of the waveform-transform circuit        1100 and to minimize the average power of output signal        S_(I)+jS_(Q) of ADC 111 a or 111 b;    -   (c) switching ADC enhancer circuits 1011 a and 1011 b to a        medium-gain mode;    -   (d) redoing waveform matching (i.e. adjusting the control        parameters of the waveform-transform circuit 1100) to minimize        the average power of S_(I)+jS_(Q);    -   (e) reconstruct the signal V_(I)+jV_(Q) as:        V _(k,I) =D _(k,I)−δ_(I) +S _(k+τ,I) /A _(I) and V _(k,Q) =D        _(k,Q)−δ_(Q) +S _(k+τ,Q) /A _(Q)  (8)

and;

-   -   (f) switching ADC enhancer circuits 1011 a and 1011 b to a        high-gain mode, and repeating steps (d) and (e) periodically.

The precision or ENOB enhancement of the reconstructed signal given byequation (8) is approximately (20 log₁₀A)/6 bits, with (20 log₁₀A theenhancer gain in dB. Enhanced ADCs (e.g., enhanced ADCs 911) directlygenerate two complex-valued digital signals expressed as{tilde over (Y)} _(k+τ)=(D _(k,I) +jD _(k,Q))−(δ_(I) +jδ _(Q))  (9){tilde over (B)} _(k) ={tilde over (Y)} _(k)(S _(k,I) /A _(I) +jS _(k,Q)/A _(Q))  (10)where {tilde over (Y)}_(k) is a delayed copy of the frequency-offsetcorrected output in FIG. 11 (e.g., output value of integer delay block1106), and {tilde over (B)}_(k) accurately digitizes the quadraturedown-converter output. {tilde over (Y)}_(k) and {tilde over (B)}_(k) area pair of matched signals. After removing the 1/Q imbalance and DCoffsets of quadrature down-converter 110, corrected signals Y_(k) andB_(k) are obtained. From statistical samples of Y_(k)/B_(k), the signalanalyzer can measure the conditioned expectationsζ_(m) =E{(Y _(k) /B _(k))|r _(m) <|B _(k) |<r _(m) +Δr}, for r_(m)=(m−0.5)Δr,m=1,2, . . .  (11)which may be used to update the values in the LUT circuits in DPDprocessor 106 to compensate for the memory-less non-linearity in poweramplifier 108. As for the non-linear memory effects, DPD processor 106minimizes the out-of-band emission of RF feedback signal 115.

As discussed above, initially, ADC enhancer circuits 1011 a and 1011bare at a low-gain mode. In this initial stage, digital pre-distortiontechniques can only linearize power amplifier 108 to a point withrelatively poor linearity, due to the coarse precision of ADCs 111 a and111 b. In the next step, ADC enhancer circuits 1011 a and 1011 b areswitched to a medium-gain mode, and the enhanced precision of ADC 111 aand 111 b improves the pre-distortion. Then, the enhancer circuits 1011a and 1011 b are switched to a high-gain mode and the output signal ofpower amplifier 108 can achieve further a higher linearity. Themaximum-allowed ADC enhancer circuit gain is limited by residualnon-linear distortions that cannot be removed by digital pre-distortiontechniques. Lab experiments have shown that, for a large variety ofpower amplifiers, the DPD methods of the present invention allow for30-dB or higher ADC enhancer circuit gain. Therefore, the ADC enhancercircuit of the present invention can provide ENOB enhancement of 5-bitor more.

The ADC enhancer circuits of the present invention lower the requirementon ADCs. As a result, power efficient 6-bit ADCs may be selected with a˜5.5-bit ENOB. For example, time-interleaved successive approximationADCs or folding-flash ADCs may be used. Such ADCs may operate at asampling rate of 1000 MS/s in 65-nm CMOS. Enhanced ADCs of the presentinvention may provide an ENOB of 10.5 bit or higher at significantlylower power than conventional pipelined ADCs.

The down-converted RF feedback signal from power amplifier 108 includesa large known signal component. The present invention provides amixed-signal processing method (the ADC enhancer circuit) that canseparate the known signal from the residual distortion and noise. Theanalog-to-digital (A/D) conversion of the distortion plus noise is mucheasier than direct A/D conversion of RF feedback signal 115. Theundesired distortion may be suppressed by digital pre-distortiontechniques. The present invention also provides a novel DPD method thatperforms well in conjunction with the ADC enhancement technique.

FIG. 12 is a block diagram showing two-stage ADC enhancer circuit 1200,in accordance with one embodiment of the present invention. ADC enhancercircuit 1200 may be used in each of I and Q channels. First-stage 1201of ADC enhancer circuit 1200 applies the current-steering techniquediscussed above in conjunction with FIG. 10. Cascaded T/H circuits 1214and 1215 driven by complementary clock signals serve as a master-slavesample-and-hold block that holds analog samples for each clock period.Second stage 1202 of ADC enhancer circuit 1200 uses acharge-redistribution technique to acquire each analog voltage sample,which is then subtracted from a voltage from a voltage output DAC 1216.The difference signal is amplified in amplifier 1218 and then providedto an ADC, such as conventional ADC 111 discussed above. Thus, withanalog input signal V(t) and digital input signal D_(k), an enhanced ADCof the present invention incorporating ADC enhancer circuit 1200provides:S _(k+τ) =A ₂ A ₁ [V(t _(k))−D _(k) ⁽¹⁾ ]−A ₂ D _(k+1) ⁽²⁾ +A ₂ A ₁ δ+q_(k)  (12)where D_(k) ⁽¹⁾ and D_(k) ⁽²⁾ (i.e., 1221 and 1222) are the inputsignals from digital processing circuit 1220 to first and second stages1201 and 1202, respectively; and A₁ and A₂ are the gains of amplifiers1213 and 1218. The parameters (A₁, A₂, δ) can be accurately identified30 from calibrations. In one embodiment, signal D_(k) is provided as:D _(k) ⁽¹⁾ =A ₁·round(D _(k) /A ₁),D _(k) ⁽²⁾ =A ₁(D _(k−1) −D _(k−1)⁽¹⁾)  (13)such that:S _(k+τ) =A ₂ A ₁ [V(t _(k))−D _(k) +δ]+q _(k)  (8)which is identical to the value of S_(k+r) obtained from equation (7),when the ADC enhancer circuit gain A=A₁A₂.

The enhanced ADCs of the present invention can operate at a samplingrate of 1000 MS/s or higher. Thus, signal analyzer 902 can supportnon-zero intermediate-frequency down-conversion for RF signals with anoriginal bandwidth up to 100 MHz. In one embodiment, only one ADCassociated with either the in-phase or quadrature component is used forsignal analysis. A double-rate or quadruple-rate up-sampler maypreferably be inserted, for example, between complex-gain multiplier1103 and frequency-offset multiplier 1104 of FIG. 11, as theintermediate frequency is typically ˜2.5 times the bandwidth of theoriginal RF signal. If a quadruple-rate up-sampler is provided, the ADCclock rate is also preferably the quadruple rate.

The above detailed description is provided to illustrate the specificembodiments of the present invention and is not intended to be limiting.Numerous modifications and variations within the scope of the presentinvention are possible.

The present invention is set forth in the accompanying claims.

We claim:
 1. A system comprising: a crest-factor reduction circuit thatreduces a crest factor of a baseband signal and that generates afeedforward signal; a signal analyzer that generates parameters based onthe feedforward signal and an output signal from a power amplifier; apre-distortion circuit that generates a pre-distorted baseband signalbased on the parameters for input to the power amplifier; and anover-sampler that increases a data rate of the feedforward signal by atleast a factor of
 2. 2. The system of claim 1 wherein the pre-distortioncircuit generates the pre-distorted baseband signal by applying amemoryless nonlinear gain and a polynomial function to the basebandsignal.
 3. The system of claim 2 wherein the memoryless nonlinear gainis applied after a unit delay to the baseband signal.
 4. The system ofclaim 1 further comprising: an up-converter that converts thepre-distorted baseband signal based on a timing signal to apre-distorted signal input to the power amplifier, wherein the signalanalyzer comprises: a downconverter that converts the output signal fromthe power amplifier based on the timing signal to a first signal that isat baseband or at a near-baseband intermediate frequency; ananalog-to-digital converter that converts the first signal to a secondsignal; and a signal analysis circuit that generates the parametersbased on the second signal and the feedforward signal.
 5. The system ofclaim 4 wherein the signal analysis circuit performs one or more of DCoffset correction, frequency offset correction, I/Q imbalancecorrection, delay matching, and gain adjustment on one or more of thesecond signal and the feedforward signal.
 6. The system of claim 4wherein the signal analysis circuit generates the parameters based onminimizing a difference between the second signal and the feedforwardsignal.
 7. The system of claim 2 wherein the polynomial function isbased on the baseband signal and a delayed version of the basebandsignal and comprises a power of a modulus of the baseband signal and apower of a modulus of the delayed version of the baseband signal.
 8. Asystem comprising: a crest-factor reduction circuit that reduces a crestfactor of a baseband signal and that generates a feedforward signal; asignal analyzer that generates parameters based on the feedforwardsignal and an output signal from a power amplifier; and a pre-distortioncircuit that generates a pre-distorted baseband signal based on theparameters for input to the power amplifier, wherein the pre-distortioncircuit generates the pre-distorted baseband signal by applying amemoryless nonlinear gain and a polynomial function to the basebandsignal, and wherein the polynomial function is based on the basebandsignal and a delayed version of the baseband signal and comprises apower of a modulus of the baseband signal and a power of a modulus ofthe delayed version of the baseband signal.
 9. The system of claim 8wherein the memoryless nonlinear gain is applied after a unit delay tothe baseband signal.
 10. The system of claim 8 further comprising anover-sampler that increases a data rate of the feedfoward signal by atleast a factor of
 2. 11. The system of claim 8 further comprising: anup-converter that converts the pre-distorted baseband signal based on atiming signal to a pre-distorted signal input to the power amplifier,wherein the signal analyzer comprises: a downconverter that converts theoutput signal from the power amplifier based on the timing signal to afirst signal that is at baseband or at a near-baseband intermediatefrequency; an analog-to-digital converter that converts the first signalto a second signal; and a signal analysis circuit that generates theparameters based on the second signal and the feedforward signal. 12.The system of claim 11 wherein the signal analysis circuit performs oneor more of DC offset correction, frequency offset correction, I/Qimbalance correction, delay matching, and gain adjustment on one or moreof the second signal and the feedforward signal.
 13. The system of claim11 wherein the signal analysis circuit generates the parameters based onminimizing a difference between the second signal and the feedforwardsignal.
 14. A system comprising: a downconverter that converts an outputsignal from a power amplifier to a first signal that is at baseband orat a near-baseband intermediate frequency; an analog-to-digitalconverter that converts the first signal to a second signal; a signalanalysis circuit that generates parameters based on the second signaland a feedforward signal representative of a baseband signal; ananalog-to-digital converter enhancer circuit that receives from thesignal analysis circuit a third signal representing a clock period ofthe feedforward signal; and a pre-distortion circuit that generates apre-distorted baseband signal based on the parameters for input to thepower amplifier.
 15. The system of claim 14 further comprising acrest-factor reduction circuit that reduces a crest factor of thebaseband signal and that generates the feedforward signal.
 16. Thesystem of claim 14 wherein the pre-distortion circuit generates thepre-distorted baseband signal by applying a memoryless nonlinear gainand a polynomial function to the baseband signal.
 17. The system ofclaim 16 wherein the memoryless nonlinear gain is applied after a unitdelay to the baseband signal.
 18. The system of claim 16 wherein thepolynomial function is based on the baseband signal and a delayedversion of the baseband signal and comprises a power of a modulus of thebaseband signal and a power of a modulus of the delayed version of thebaseband signal.
 19. The system of claim 14 further comprising anover-sampler that increases a data rate of the feedfoward signal by atleast a factor of
 2. 20. The system of claim 14 further comprising: anup-converter that converts the pre-distorted baseband signal based on atiming signal to a pre-distorted signal input to the power amplifier,wherein the downconverter converts the output signal from the poweramplifier to the first signal based on the timing signal.
 21. The systemof claim 14 wherein the analog-to-digital converter enhancer circuitcomprises: a digital-to-analog circuit that converts the third signalfrom the signal analysis circuit to a first output; and a summer thatgenerates a second output based on the first output and the first signalfrom the downconverter, wherein the second output is input to theanalog-to-digital converter.
 22. The system of claim 14 wherein theanalog-to-digital converter enhancer circuit comprises: a firstdigital-to-analog circuit that converts the third signal from the signalanalysis circuit to a first output; a first summer that generates asecond output based on the first output and the first signal from thedownconverter; cascaded first and second tracking-and-hold circuitsoperated by complementary clock signals that generate a held signalbased on the second output; a second digital-to-analog converter thatconverts the third signal from the signal analysis circuit to a thirdoutput; and a second summer that generates a fourth output based on theheld signal and the third output, wherein the fourth output is input tothe analog-to-digital converter.